No ruler/protractor/interactive ruler hotkey?
Maybe not, but you could create one easily enough.  In the Toolbar, in the Grease Pencil tab, right-click on the "Ruler/Protractor" button.  From the menu that pops up, choose "Add Shortcut".  Then make it whatever you want, e.g. Ctrl-Alt-R (I don't think that's mapped to anything?).